Reactance-free reversely folded resistive shunt in coaxial line



REACTANCE-FREE REVERSELY FOLDED RESISTTVE SHUNT IN COAXIAL LINE Filed March 8, 1967l United States Patent O 3,458,846 REACTANCE-FREE REVERSELY FGLDED RESISI'IVE SHUNT IN COAXIAL LINE Malcolm W. De Young, Edmonds, Wash., assgnor to John Fluke Mfg. Co., Inc., Snohomish, Wash., a corporation of Washington Filed Mar. 8, 1967, Ser. No. 621,507 Int. Cl. H01c 3/02 U.S. Cl. 338-61 4 Claims ABSTRACT F THE DISCLOSURE A reactance-free characteristic is achieved in a lowresistance shunt device by the reversely folded configuration of the shunts resistance element providing self-neutralization of self-reactance effects and, additionally, permitting connection of its free ends to the respective transmission line conductors at longitudinal locations which substantially correspond. Consequently the opposed input line conductors which deliver signal current to the shunt junction points are of substantially equal lengths and are thereby mutually compensatory insofar as net reactance effect is concerned. The remaining uncompensated length of line conductor (the center conductor in the coaxial line application) is located between the shunts junction points and its output terminals, such that the attendant line conductor self-inductive reactance, being in series with the much higher impedance of the meter or other load device, becomes negligible.

The present invention comprises a reactance-free resistive shunt for metering devices and other electrical loads having an impedance which is high in relation to shunt resistance. An object hereof is to provide a shunt device having an electrically balanced input and a shunt conductance path which is purely resistive, such that the ratio of signal current division occurring between such path and the output circuit is independent of signal frequency. The invention is herein illustratively described by reference to the presently preferred form thereof in its coaxial transmission line application; however, it will be recognized that certain modifications and changes therein may be made without departing from the essential features involved.

In the prior art technique the shunts resistive element, of tubular form and approximately of the diameter of the coaxial line outer conductor, was interposed physically between outer line conductor input and output sections. The connection points between the respective ends of the tubular resistance element and the coaxial line inner and outer conductors were therefore longitudinally separated by substantially the full length of the resistive element. This arrangement resulted in uncompensated inductive reactance in the intervening length of coaxial line inner conductor which was electrically in series with the shunts resistance element. Consequently impedance of the shunt path through the device varied significantly with applied signal frequency owing to the normal relatively low-resistance value of the resistance element. Also this arrangement produced an electrically unbalanced input.

A specific object of this invention is to provide a broadband resistive shunt which overcomes the aforementioned problems and shortcomings.

As herein disclosed the .improved shunt device employs a tubular resistive element of tubular form which is folded back upon itself substantially in hal-f so that it provides bifilar effect reactance compensation within itself and, more importantly, permits connecting `its free ends to the inner and outer line conductors at locations correspondingly located along the length of the device. Consequently the shunt device transmission line input includes effec- 3,458,846 Patented July 29, 1969 ICC tively equal lengths of inner and outer conductors which electrically balance the input and eliminate uncompensated inductive reactance in the conductive shunt path through the device.

These and other features of the invention will become more fully evident from the following description by reference to the accompanying drawings.

FIGURE 1 is a schematic diagram of the shunt with a meter connected across its output.

FIGURE 2 is a side view partly in section of the irnproved shunt in its presently preferred coaxial line embodiment.

FIGURE 3 is a side view partly in section of another coaxial line embodiment of the improved shunt.

In FIGURE 1 the shunt S comprises the line conductors 10 and 12 having input terminals x14 and 16 and output terminals 18 and 20, with a resistance element 22 serving as the shunt conductor interconnecting the line conductors at junctions 24 and 26 intermediate the input and output terminals. The shunt is intended for use with a meter M or other load device connected across output terminals 18 and 20 and having an electrical impedance Z which is high in relation to the shunt resistance 22 and also in relation to any reactance developed in the shunt S within the frequency band of signals to be impressed on input terminals 14 and 16. In accordance with this invention reactance is neutralized in the line conductors 10 and 12 ahead of junctions 24 and 26; also in the resistance element 22 and its leads extending between junctions 24 and 26. As will become evident hereinafter, with the novel construction any residual line conductor reactance is transposed to the meter or load circuit leads, i.e. behind junctions 24 and 26 where it is in series with the meter impedance Z. Since this uncompensated reactance is small in relation to meter impedance Z its effect on the division of signal current between the respective paths through shunt resistance 22 and the meter, hence its effect on constancy of the shunts pure resistance characteristic as a function of frequency is negligible. It will be noted that uncompensated reactance left in either of the line conductors 10 and i12 ahead of the junctions 24 and 26 would cause the shunt to present an unbalanced load to a signal source (not shown) connected across input terminals 14 and 16. More importantly uncompensated reactance left in the resistance element 22 or its connecting leads extending between junctions 24 and 26 would, even though of a magnitude which is small in relation to meter impedance Z, cause the division of signal current between the Iaforementioned two ypaths to vary significantly with change of signal frequency owing to the fact that the shunt resistance 22 is also small in its order of magnitude. Therefore in the schematic image of FIGURE 1, the invention provides a broadband shunt device which substantially eliminates reactance effects ahead of the junctions 24 and 26 and through the resistance element 22 and its connecting leads by an arrangement which transfers any residual `or uncompensated reactance to the portion of line conductors 10 and 12 extending from junctions 24 and 26 to the output terminals 18 and 20.

In the specific embodiment of FIGURE 2 physical parts and circuit elements which correspond to those shown schematically in FIGURE l bear corresponding reference numerals. Line conductors 10 and 12 respectively comprise the outer and inner conductors of a length of coaxial transmission line. Input and output terminals 14 and 18 for the tubular outer conductor 10 comprise fittings 10a and 10b formed in conventional manner for connection to external coaxial line sections by threading the ends of the respective fittings. Similarly input and output terminals 16 and 20 for the rod-like inner line conductor 12 are formed in conventional manner for connection to external coaxial 3 line sections simply by severing and slightly chamfering the conductor ends approximately in the respective termini planes of the outer line conductor 10.

Electrical and physical continuity between outer line conductor fittings 16a and 10b is provided by a thin tubular conductive shell c serving as a ground shield, the respective ends of which externally overlap the inner ends of such fittings and are silver soldered or otherwise conductively bonded thereto. In addition, fittings lila and 10b have annular external fianges 10a and 10b'. A thick tubular metal jacket 28 of externally ribbed configuration surrounds the shell 10c and, with its ends abutted and thermally bonded to the respective flanges 10a and 10b', serves as a heat sink and radiator for the shunt.

Resistance element 22 is of thin reversely folded tubular configuration having the free end of its outer fold abutted and electrically bonded at junction 24 to the inner end of input fitting 10a of outer line conductor 10. The free end of the inner fold of tubular resistance element 22 overlaps and is bonded to the periphery of conductive disk 30 mounted and electrically bonded to the inner line conductor 12 at junction 26. An insulative sleeve 31 separates the resistance element folds, and an insulative sleeve 32 separates the shell 10c and the outer fold of the resistance element. A sleeve 34 surrounding the inner conductor 12 insulates it from the resistance elements inner fold. By providing resistance element folds of approximately equal lengths the junctions 24 and 26 are located near each other longitudinally of the shunt device, so that resistance element 22 is served from input terminals 14 and 16 by effectively equal lengths of inner and outer coaxially line conductors 10 and 12. Because of this arrangement reactance effects in the input of the shunt device are neutralized and the inherent electrical symmetry of the input of a balanced coaxial line is not impaired by the presence of the resistance element incorporated coaxially therein. Moreover, the bifilar effect of the folded resistance element 22 eliminates reactance effects in the electrical path which extends through this element between junctions 24 and 26.

However, it will be noted that there remains between junction 26 and output terminal 20a substantial length of transmission line 1012 along which the electrical proximity between the inner conductor 12 and the inner fold of the resistance element differs materially from that between the outer conductor 10 (10c) and the resistance elements outer fold. This difference results in a net inductive reactance in the inner line conductor between the output terminals 18, and the junctions 24, 26. However, because this reactance is small in relation to meter (load) impedance Z and is in series therewith it becomes negligible in the operation of the shunt.

In the modified shunt device appearing in FIGURE 3 parts similar to those in the preceding embodiment bear corresponding reference numerals. This modified device is essentially similar to that of FIGURE 2; however, it dispenses with the shell 10c interconnecting the outer conductor fittings 10a and 10b. Instead it relies upon the jacketing heat sink 28 as the intermediate section of coaxial line outer conductor. Therefore this modilied version is somewhat simpler than the first, but it lacks the first embodiments advantage of providing an electrically cornplete unit which can be assembled and production-tested prior to installation of the jacketing heat sink. In order to permit staging the final assembly in this manner in the case of FIGURE 2 one (or both) of the outer line fitting flanges, such as ange 10b', comprises a separate ring which may be slipped over the fitting 10b and bonded in position after the heat sink is in place. In FIGURE 3 both of the flanges may be integral parts of their respective outer line fittings as shown. The heat sink then serves as in the first embodiment to absorb and radiate the heat generated in shunt resistance 22 along which it is closely positioned.

I claim as my invention:

1. A broad-band resistive shunt device comprising an input, an output, generally elongated coaxial inner and outer liner conductors extending between said input and output, an elongated resistance element of reversely folded tubular configuration received between and coaxial with said line conductors at a location intermediate said input and output, said resistance element having free ends physically located at substantially corresponding locations along said line conductors and directly connected electrically to the line conductors respectively at said locations.

2. The shunt device defined in claim 1, wherein the outer coaxial line conductor comprises longitudinally spaced input and output line sections electrically interconnected by a conductive tubular ground member closely surrounding the tubular resistance element.

3. The shunt device defined in claim 2, wherein the conductive tubular ground member comprises a heat sink which jackets the tubular resistance member and which is thick in relation to the latter.

4. The shunt device defined in claim 2, and a relatively thick conductive tubular jacket surrounding the conductive tubular ground member and separately joined by its ends respectively to the input and output line sections.

References Cited UNITED STATES PATENTS 2,137,787 1l/1938 Snow 338-61 X 2,521,894 9/1950 Brown 338--61 2,568,600 9/1951 Wirk 338-61 X HERMAN KARL SAALBACH, Primary Examiner PAUL L. GENSLER, Assistant Examiner U.S. Cl. X.R. S33-81; 338-216 

